1. Field of the Invention
The present invention relates to a method and apparatus for detecting an impact on an automotive vehicle, and the apparatus for detecting the impact to actuate a vehicle passenger restraint system, and more particularly to the method and apparatus using a wavelet function provided on the basis of a mother wavelet function which is localized in time, for use in the restraint system such as an air bag system, a seat belt pre-loader or the like.
2. Description of the Prior Arts
Recently, an air bag system is installed in a vehicle as a restraint system to supplement a seat belt. That is, the air bag is provided for lessening the shock to a driver in the event of a frontal an impact on the vehicle exceeding a predetermined level. According to a conventional air bag system, when an acceleration sensor senses the frontal an impact greater than a predetermined threshold level, an air bag inside a steering wheel is momentarily inflated to reduce the shock to the driver. In this restraint system, it is important to detect the impact on the vehicle accurately and rapidly. Therefore, improvement of an acceleration sensor is required, and also required is improvement in a method and apparatus for detecting an impact force, a condition of the impact, or the like, accurately in response to an acceleration signal sensed by the acceleration sensor. In Japanese Patent Laid-open publication No.4-358945, for example, it is proposed to detect a collision accurately even in the case where output of the acceleration signal is delayed largely, in such a case as oblique an impact, pole an impact, or the like. There is proposed an actuator for a vehicle restraint system, wherein an integral value of the acceleration signal for a predetermined period of integration and a differential value of the acceleration signal at a predetermined time for the period of integration are summed to determine the impact in response to the result of summation.
In Japanese Patent Laid-open Publication No.6-211100, a vehicle collision detecting method is proposed to detect the impact rapidly and accurately, by means of the steps of obtaining a short term integral value and a long term integral value of an output of an acceleration sensor, computing an impact force by sampling its component in a particular band especially caused in case of a vehicle collision and square-multiplying the component, and then determining the impact when both of the impact force and the short term integral value exceed predetermined threshold levels, respectively, or when the long term integral value exceeds a predetermined threshold level, thereby to determine the impact totally on the basis of the amount of the velocity varied during the short and long terms along with the impact force.
In the U.S. Pat. No. 5,185,701, it is proposed to provide a method for distinguishing between different types of vehicle crashes by determining which frequency components are present in a signal from a deceleration sensor upon the occurrence of a vehicle crash condition. Also, in the U.S. Pat. No. 5,034,891, it is proposed to provide a method and apparatus for electrically controlling an actuatable passenger restraint system which includes filter means connected to sensing means for providing a signal having a value when the sensing means provides a signal that includes particular frequency components. Furthermore, in the U.S. Pat. No. 5,065,322, it is proposed to provide a method and apparatus for electrically controlling an actuatable passenger restraint system which actuates the system only upon a frequency domain summation algorithm indicating the occurrence of a predetermined type of crash. It is proposed in the U.S. Pat. No. 5,036,467 to provide a method and apparatus for electrically-controlling an actuatable passenger restraint system which actuates the system only upon a frequency domain integration and summation algorithm indicating the occurrence of a predetermined type of crash. As one aspect of the invention disclosed in the last United States Patent for example, a method is provided for controlling actuation of a passenger restraint system in a vehicle. The method includes the steps of providing a time domain vibratory electric signal having frequency components indicative of a vehicle crash condition, transforming the time domain vibratory electric signal over at least two time intervals into associated frequency domain signals, integrating each of the frequency domain signals, summing the integrals of the frequency domain signals, and actuating the passenger restraint system when the sum of the integrals of the frequency domain signals indicates a predetermined type of vehicle crash is occurring.
According to the above-described methods and apparatuses for detecting the impact, however, the differential value or integral value of the acceleration (deceleration), the sum or integral value of the particular frequency or the like has been used, so that the time component may cause changes in various conditions such as the direction of the impact, to result in an error or delay in detecting the impact, which can not be recovered to ensure a desired property by an ordinary way of recovery such as a way for eliminating a noise. Yet, it is extremely difficult to determine a condition of the impact on the vehicle. In the method and apparatus as described in the United States Patents listed above, the frequency component of the vibratory electric signal caused in collision is to identified. Thus, as far as a target to be controlled includes the frequency component, it is difficult to cancel any noise completely, so that it may cause a delay in time when determining the impact, depending upon the condition of the impact. Even if well known Fourier transform was used for analyzing the acceleration signal, it would be difficult to measure accurately the time when a particular frequency component generates, so that it would be difficult to provide an accurate time for inflating the air bag.
In a field of analyzing signals, the Fourier transform has been utilized in many occasions. In order to divide or combine those signals, a wavelet transform is getting popular recently for use in various fields such as audio, display or the like, as disclosed in Japanese Patent Laid-open Publication No. 4-275685. It is known that the wavelet transform is a method for dividing an input signal into wavelets as its components, and re-configuring the original input signal as a linear coupling of the wavelets. The wavelet transform is effectively used for analyzing an unsteady state, such as state transition or the like, and has as its base a mother wavelet function, to which a scale transform and a shift transform are performed. The mother wavelet function is a square-integrable function which is localized in time, and the base of which may be selected freely as far as flexible admissible conditions can be met, although the base has to be the one to be bounded, or the one to be attenuated rapidly in a remote area. Furthermore, the mother wavelet function may be used effectively for identifying a position of a singular point, because it has various characteristics, such that the base is analogous, that the direct current component is not included, and that a dissector rate for analysis can be provided freely.
In the meantime, it is so arranged that when the impact is applied to the vehicle in collision, vibration caused by the impact is transmitted to an acceleration sensor which outputs a signal corresponding to the impact force. This output signal has a repeatability depending upon structure of the vehicle, e.g. a front structure of the vehicle in case of a frontal collision. Therefore, if a singular characteristic can be found by analyzing the output signal, conditions of the impact (e.g., direction, magnitude or the like of the impact) can be made clear.